In oil well drilling, as well as in penetrations of the earth for other reasons, directional bores (other than straight) are often drilled to recover oil from inaccessible locations; to stop blowouts; to sidetrack wells; to by-pass broken drill pipe; and for various other reasons.
Conventional techniques for directional drilling in wells use a deflector in the borehole to push the bit sideways (e.g. "whipstocking"); or alternatively insert a bent joint in the drilling string (e.g. "bent subs"); or alternatively propell pressurized drill mud sideways through a nozzle in the drill to push the bit sideways (e.g. "side jetting").
The "whipstocking" process requires a series of separate operations including drilling of a pilot hole, reaming of the pilot hole to full gauge, and removal of the deflector, and is therefore a time consuming and costly process. The use of "bent subs" to produce lateral forces on the drill bit requires the use of expensive drill motors; and the "side jetting" process, using special drill bits to provide offset holes by the pressurized drill mud, does not function well in hard rock earth since the conventional mud pressures will not erode the hard rock materials.